1. Technical Field
This invention relates to the field of semiconductor processing. More specifically, the invention relates to the repair of masks to promote adhesion of patches.
2. Background Art
Phase shift masks are masks, generally made of quartz, that are partially covered with phase shift material and partially clear. If the phase shift material is xe2x80x9cattenuatedxe2x80x9d phase shift material, the attenuated phase shift material causes a reduction in light energy to a level beneath that required to cause positive photoresist, for example, to become removable. On the other hand, light passing through the clear areas is at a high enough energy that the photoresist becomes removable. The attenuated phase shift material changes light passing through it to be out of phase with light passing through the clear areas. At the junction between a phase shift material and a clear area, a reduction in light energy occurs due to cancellation caused by the differences in phase between light passing through the phase shift material and through the clear area. This cancellation of light energy causes an improvement in edge resolution. In other words, the edges are much more sharply defined and steeper than what would otherwise result.
A xe2x80x9cclear defectxe2x80x9d on a phase-shift mask is an area where there is no phase shift material, but where there should be phase shift material. Generally, clear defects are caused by imperfections in the photoresist that is used to define the areas on the mask of attenuated phase-shift material. Clear defects are usually patched, as if the clear defects are not patched, the defects will essentially be replicated (will xe2x80x9cprintxe2x80x9d) in the photoresist. To patch clear defects, Focused Ion Beam(FIB) milling of the defect areas is generally used, and is usually followed by FIB deposition of patch material. However, under certain conditions, all or some of the patch can come off the reticle""s surface. If any part of the patch comes off, the part no longer patched will be printed on the photoresist as a defect.
What is needed is a way of repairing phase shift masks to promote adhesion of clear defect patches and to thereby reduce the propensity for patches on phase shift masks to lift off the reticle surface.
Before proceeding to important aspects of the current invention, it is important to understand at this point simple descriptions of the problems associated with clear defect patches. Consequently, a more detailed analysis of the problem will be discussed. Attenuated phase shift material is usually etched through a dry etch, and this etching can leave residue (illustratively, fluorine). Prior to milling of clear defects with a Focused Ion Beam (FIB), a conductive layer (usually copper) is added to the surface of the reticle. This conductive layer discharges the charge caused by the FIB, thereby preventing drifting and improving accuracy. Once the patches have been deposited, the conductive layer is removed by etching (illustratively, in nitric acid). The defect patches tend to lift off the surface of masks because etchant interacts with any residue remaining at the interface of the mask""s substrate and the patch and acts as a solvent for the patch. The solvent in this situation tends to leach underneath the patch and lift off the patch.
To overcome these problems, the current invention performs short pulse laser ablation of clear defect regions on a mask prior to patching the clear defect regions. The short pulse laser ablation removes any residue that absorbs light. Thus, the ablation completely cleans the surface of the clear defect regions, meaning that any patches of the surface will better adhere to the surface of the mask. This is particularly important during those situations where a later etch of a conductive surface added to the mask or any later cleaning stage creates a solvent because the etchant interacts with residue on the mask, and wherein the solvent attacks the patch material at the patch material""s interface.
The foregoing and other features and advantages of the present invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.